Nowadays, for reasons of environmental conservation, research and development of clean energy is being vigorously pursued. Among such energy sources, solar cells are attracting considerable attention due to the limitless nature of the sunlight that acts as the resource, and the fact that the sunlight is non-polluting. Conventionally, electric power generation from sunlight using solar cells has often used monocrystalline or polycrystalline silicon. However, with these silicon materials used in solar cells, because a great deal of energy and time is required for crystal growth, and complex steps are also required in the subsequent production process, improving the mass production efficiency is difficult, and providing a low-cost solar cell is problematic.
In contrast, so-called thin-film semiconductor solar cells (hereafter referred to as thin-film solar cells) that use a semiconductor such as amorphous silicon can be produced by forming the required quantity of a semiconductor layer that functions as the photovoltaic layer on top of a low-cost substrate such as glass or stainless steel. Accordingly, it is thought that because these thin-film solar cells are thin and lightweight, cheap to produce, and easily produced with large surface areas, they are likely to become the predominant type of solar cell.
However, thin-film solar cells have a lower conversion efficiency than solar cells that use crystalline silicon, and have therefore not yet become widely used. As a result, a variety of innovations are currently being used to improve the performance of thin-film solar cells.
One of these innovations involves improving the light reflection properties from the back surface of the photovoltaic layer, namely, from the back electrode of the thin-film solar cell, which represents one of the potential fields of application for the present invention. This enables sunlight that has not been absorbed by the photovoltaic layer to be reflected back into the photovoltaic layer, meaning sunlight that has conventionally not been absorbed can be used more effectively.
Of the various possibilities, in order to enable the photovoltaic layer to efficiently absorb light from the low-energy, long wavelength region, the formation of a surface structure having unevenness with dimensions of several tens of nanometers to several microns, a so-called textured structure, on the back electrode has proven extremely effective. Light that has not been absorbed by the photovoltaic layer and reaches the back electrode is subjected to scattered reflection at the back electrode having this textured structure, and re-enters the photovoltaic layer in a changed direction. This light scattering lengthens the light path, and by ensuring total reflection conditions, ensures that the light is effectively confined within the solar cell. This effect, which is known as the “optical confinement effect”, promotes light absorption within the photovoltaic layer, thereby improving the conversion efficiency of the solar cell. This optical confinement effect has become an essential technique in improving the conversion efficiency of solar cells.
As illustrated in FIG. 5, in a super straight-type solar cell 110 in which light enters from the side of a transparent substrate, the solar cell usually adopts a structure in which a transparent electrode 112, a photovoltaic layer 113 consisting of an amorphous Si layer 113a and a microcrystalline Si layer 113b, and a back electrode 115 are formed in sequence on a substrate 111. In order to achieve light scattering and the optical confinement effect, a textured structure 112a is generally formed on the light incident-side transparent electrode 112, which is a material such as SnO2 for example, and the optical confinement effect is realized by generating light scattering at this textured structure. In this super straight-type solar cell, in order to achieve surface passivation of the photovoltaic layer 113, ohmic contact with the back electrode 115, and an optical design having increased reflectance, a transparent conductive film 114 is formed between the photovoltaic layer 113 and the back electrode 115.
On the other hand, as illustrated in FIG. 6, in the case of a substrate-type solar cell 120 in which light enters from the surface of a photovoltaic layer, the solar cell usually adopts a structure in which a back electrode 122, a photovoltaic layer 123 consisting of an amorphous Si layer 123a and a microcrystalline Si layer 123b, and a transparent electrode 124 are formed in sequence on a substrate 121. A textured structure 122a is generally formed on the back electrode 122 to generate light scattering, thereby realizing the optical confinement effect.
Examples of methods that have been proposed for forming the back electrode having a textured structure within this type of substrate-type solar cell include: methods in which a metal film is converted to a polycrystalline form by conducting heating during vapor deposition (for example, see Patent Document 1), methods in which sputter etching is conducted following vapor deposition and heat treatment of the metal electrode (for example, see Patent Document 2), methods in which localized silver aggregation is promoted by conducting heating during vapor deposition, thereby forming a semi-continuous film having surface texture, and then conducting a vapor deposition of silver at low temperature to form a continuous film (for example, se Patent Document 3), methods in which a textured film is formed by vapor deposition of an alloy such as Al—Si while the substrate is subjected to heating, and a metal film having a high reflectance is then formed by vapor deposition on top of the textured film (for example, see Patent Document 4), and methods in which a textured film is formed by vapor deposition of a Ag—Al alloy (for example, see Patent Document 5).    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. Hei 03-99477 (page 6 upper left column line 19 to page 6 upper right column line 3)    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. Hei 03-99478 (Claim 1))    Patent Document 3: Japanese Unexamined Patent Application, First Publication No. Hei 04-218977 (Claim 2, paragraphs [0019] to [0020], and FIG. 1)    Patent Document 4: Japanese Unexamined Patent Application, First Publication No. Hei 04-334069 (paragraph [0014])    Patent Document 5: Japanese Unexamined Patent Application, First Publication No. 2005-2387 (paragraph [0062])